The increasing accumulation of phosphorus, nitrogen and heavy metals discharged into the environment from agricultural, storm water run-off, wastewater treatment discharge and other sources is one of the most significant environmental challenges facing the planet. Phosphorus is also used in fertilizers, and given that phosphate mines are depleting and may be fully depleted within next 100 years, world food supplies will certainly be impacted.
Elevated phosphorus levels in surface waters leads to eutrophication, which is detrimental to aquatic life. To control eutrophication, the EPA recommends that total phosphates should not exceed 0.05 mg/L (as phosphorus) in a stream at a point where it enters a lake or reservoir, and total phosphorus should not exceed 0.1 mg/L in streams that do not discharge directly into lakes or reservoirs. To date, phosphorus removal has been accomplished with flocculation/precipitation methods that use metal salts such as ferric chloride, aluminum sulfate (alum) and calcium hydroxide (lime). In many cases, these methods require the use of polymers to enhance the precipitation and ultimate solids removal. Various methods have been detailed that utilize naturally occurring and synthesized forms of xonotlite and/or tobermorite to remove phosphorus. These materials are restricted by pH of the solution, as increasing pH causes bicarbonate ions to convert to carbonate ions, reducing the efficiency of removal. Another method of phosphorus removal is the chemical formation of struvite (ammonium magnesium phosphate hexahydrate). This process requires the introduction of a magnesium source, typically magnesium hydroxide, and is dependant on a high ammonia level as the ammonium source.
Nitrates are also of concern as increased levels in surface water and groundwater lead to undesirable levels in drinking water supplies. The current drinking water nitrate limit is 10 mg/L as nitrogen. Nitrate removal has most often been accomplished via microbiological denitrification. This process requires the availability of denitrifying bacteria in a reduced oxygen environment. The bacteria metabolize the nitrate resulting in reduction to nitrite and ultimately nitrogen gas.
Discharges of metal ions into water sources can render water non-potable as well as having adverse affects on aquatic life. In-stream water quality standards as well as National Drinking Water Standards for metals are very low. Metal ions have most often been removed from water and wastewater by flocculation/precipitation as metal hydroxides. This is typically accomplished by the addition of an alkali such as calcium hydroxide or sodium hydroxide. Typically a polymer is required to enhance flocculation and aid in solids removal. Metal ion removal is also accomplished by the use of cation exchange resins. This process typically requires a relatively clean water source that is free of suspended solids and oils and greases.
Accordingly, there is a need for compositions and methods for making the same that are highly efficient at removing phosphate, nitrate, and metal ions from aqueous solutions such as water and wastewater. It is especially preferable that the foregoing compositions are able to provide sufficient alkalinity necessary for effective phosphate removal without requiring additional pH adjustment. It is even more preferable that the foregoing compositions can be manipulated in ways to adsorb various cations and anions given that the aqueous solutions contemplated in connection with the present invention have differing contaminant profiles.
It is also highly preferred that the foregoing compositions are safe for use in removing contaminants from potable water sources and in food processing applications. It is also preferred that the foregoing compositions can recover phosphate and nitrate in a form that can be used as a fertilizer.